Netty基础02NIO文件编程

3、文件编程

FileChannel

​ 获取

• FileChannle只能在阻塞模式下使用

  
  



• 不能直接打开FileChannel,必须通过FileInputStream、FileOutputStream或者RandomAccessFile来获取FileChannel,它们都有getChannel方法

  
  



• 通过FileInputStream获取的channel只能读

  
  



• 通过FileOutputStream获取的channel只能写

  
  



• 通过RandomAccessFile是否能读写根据构造的RandomAccessFile时的读写模式决定

  
  

读取

• 会从channel读取数据跳虫ByteBuffer，返回值表示读到了多少个字节，-1表示到达了文件的末尾

  
  



• int readBytes = channel.read(buffer);

  

写入

• 写入的正确姿势如下

  
  



• ByteBuffer buffer = ...;
buffer.put(...); // 存入数据
buffer,flip(); // 切换读模式
while(buffer.hasRemaining()) {
channel.write(buffer);
}


  在while中调用channel.write方法并不一定保证一次将buffer中的全部内容写入channel

  
  

关闭

• channel必须关闭，不过调用了FileInoutStream、FileOutputStream或者RandomAccessFile的close方法会间接调用channel的close方法

位置

• 获取当前位置

  long pos = channel.position();

  




• 设置当前位置

  long newPos = ....;
channel.position(newPos);

  




• 设置当前位置时,如果设置为未见末尾

  这时读取会返回-1

  这时写入,会追加内容,但要注意如果position超过了文件末尾,再写入新内容和原来末尾之间会有空洞(00)

  
  

大小

• 使用size方法获取文件大小

强制写入

• 操作系统处于性能的考虑，会将数据缓存,不是立刻写入磁盘,可以调用force(true)方法将文件内容和元数据(文件的权限等信息立刻写入磁盘)

两个channel传输数据

public static void main(String[] args) {
String source = "word1.txt";
String target = "word3.txt";
try (
FileChannel from = new FileInputStream(source).getChannel();
FileChannel to = new FileOutputStream(target).getChannel();
) {
// 代码简洁,效率高，底层一版会调用操作系统的零拷贝优化shangxin
// 传输数据有上限，最多2G
from.transferTo(0, from.size(), to);
} catch (IOException e) {
e.printStackTrace();
}
}


优化(可传输大于2G的文件):

public static void main(String[] args) {
String source = "word1.txt";
String target = "word4.txt";
try (
FileChannel from = new FileInputStream(source).getChannel();
FileChannel to = new FileOutputStream(target).getChannel();
) {
long size = from.size();
// left 表示还有多少字节
for (long left = size; left > 0; ) {
System.out.println("position:"+(size-left)+",left:"+left);
left -= from.transferTo((size - left), left, to);
}
} catch (IOException e) {
e.printStackTrace();
}
}


Path

• JDK 引入Path和Paths类

  
  



• Path用来表示文件路径

  
  



• Paths时工具类,用来获取Path

  // 相对路径 使用usr.dir 环境变量来定位 word1.txt
Path path = Paths.get("word1.txt");
// 绝对路径
Path path2 = Paths.get("E:\0107TEST\\AkeyStart\\AkeyStart.bat");
// 绝对路径
Path path1 = Paths.get("E:/210107TEST/AkeyStart/AkeyStart.bat");
// 代表 E:/210107TEST/AkeyStart/AkeyStart.bat
Path path3 = Paths.get("E:/210107TEST/AkeyStart", "AkeyStart.bat");

  

  
  

  • **. **代表当前路径

    
    
  
  

  • ..代表了上一级路径

    
    
  
  
  
  

```java
Path path4 = Paths.get("E:\0107TEST\\AkeyStart\\AkeyStart.bat\\..\\AkeyStart - 副本.bat");
System.out.println(path4);
System.out.println(path4.normalize());
E:0107TEST\AkeyStart\AkeyStart.bat\..\AkeyStart - 副本.bat
E:0107TEST\AkeyStart\AkeyStart - 副本.bat
```


File

• 检查文件是否存在

  Path path = Paths.get("word10.txt");
System.out.println(Files.exists(path)); // true false

  




• 创建一级目录

  Path path1 = Paths.get("E:\\kms\\dgj");
Files.createDirectory(path1);

  




• 创建多级目录

  Path path1 = Paths.get("E:\\kms\\test\\dgj");
Files.createDirectories(path1);

  




• 拷贝文件

  // word10.txt没有
Path path1 = Paths.get("word1.txt");
Path path2 = Paths.get("word10.txt");
Files.copy(path1,path2);
// word2.txt有，覆盖
Path path1 = Paths.get("word1.txt");
Path path2 = Paths.get("word2.txt");
Files.copy(path1,path2, StandardCopyOption.REPLACE_EXISTING);


  ​ 如果文件存在,抛 java.nio.file.FileAlreadyExistsException

  
  



• 移动文件

  Path path1 = Paths.get("word1.txt");
Path path2 = Paths.get("word2.txt");
// StandardCopyOption.ATOMIC_MOVE 保证文件移动的原子性
Files.move(path1,path2,StandardCopyOption.ATOMIC_MOVE);

  




• 删除文件

  Path path1 = Paths.get("word2.txt");
Files.delete(path1);


  ​ 文件不存在:java.nio.file.NoSuchFileException: word21.txt

  
  



• 删除目录

  Path path1 = Paths.get("E:\\kms\\dgj");
Files.delete(path1);

  




• 遍历文件夹(访问者模式)

  public static void dirAndFile() {
try {
// 文件夹数
AtomicInteger dircount = new AtomicInteger();
// 文件数
AtomicInteger filecount = new AtomicInteger();
Files.walkFileTree(Paths.get("D:\\mavenjar"), new SimpleFileVisitor

  () {
@Override
public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
dircount.incrementAndGet();
System.out.println("dir--->:"+dir);
return FileVisitResult.CONTINUE;
}
@Override
public FileVisitResult visitFile(Path file, BasicFileAttributes attrs)
throws IOException
{
filecount.incrementAndGet();
System.out.println("file--->:"+file);
return FileVisitResult.CONTINUE;
}
});
System.out.println("dircount:"+dircount+"个文件");
System.out.println("dircount:"+filecount+"个文件夹");
} catch (IOException e) {
e.printStackTrace();
}
}


  遍历文件夹中有多少个jar

  public static void selectJar() {
try {
AtomicInteger atomicInteger = new AtomicInteger();
Files.walkFileTree(Paths.get("D:\\mavenjar"), new SimpleFileVisitor

  () {
@Override
public FileVisitResult visitFile(Path file, BasicFileAttributes attrs)
throws IOException {
if(file.toString().endsWith(".jar")){
atomicInteger.incrementAndGet();
System.out.println(file);
}
return super.visitFile(file,attrs);
}
});
System.out.println(atomicInteger);
} catch (IOException e) {
e.printStackTrace();
}
}

  




• 删除多级目录

  Files.delete(Paths.get("D:\\ruoyi")); //java.nio.file.DirectoryNotEmptyException: D:\ruoyi 有文件 删除失败


  public static void deleteDir() {
// 删除多级目录
try {
Files.walkFileTree(Paths.get("D:\\ruoyi"), new SimpleFileVisitor

  () {
@Override
public FileVisitResult visitFile(Path file, BasicFileAttributes attrs)
throws IOException {
Files.delete(file);
return super.visitFile(file, attrs);
}
@Override
public FileVisitResult postVisitDirectory(Path dir, IOException exc)
throws IOException {
Files.delete(dir);
return FileVisitResult.CONTINUE;
}
});
} catch (IOException e) {
e.printStackTrace();
}
}

  

  
  

  • 文件夹拷贝

    public static void backups() throws IOException {
String source = "D:\\代码备份";
String target = "D:\\代码备份testnio";
Files.walk(Paths.get(source)).forEach(path -> {
// 是否是目录
try {
String replace = path.toString().replace(source, target);
if (Files.isDirectory(path)) {
Files.createDirectory(Paths.get(replace));
}
// 是否是文件
else if (Files.isRegularFile(path)) {
Files.copy(path, Paths.get(replace));
}
} catch (IOException E) {
}
});
System.out.println("copy complete");
}